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Before you begin…

Download and install Titus. This article was tested with Titus 0.5.14; newer versions should work with no modification. Python >= 2.6 and < 3.0 is required.

Launch a Python prompt and import titus.prettypfa:

Python 2.7.6
Type "help", "copyright", "credits" or "license" for more information.
>>> import titus.prettypfa as prettypfa

Realistic example

The simple example applies a mathematical formula to user-supplied parameters, but PFA was designed for data processing. The example in this section is more realistic.

To begin, download the Exoplanets dataset, which we use for examples. It can be read with Avro or fastavro.

>>> from avro.datafile import DataFileReader
>>> from avro.io import DatumReader
>>> exoplanetsIter = DataFileReader(open("exoplanets.avro"), DatumReader())
>>> exoplanets = list(exoplanetsIter)
>>> print len(exoplanets)
1103

or

>>> import fastavro
>>> exoplanetsIter = fastavro.reader(open("exoplanets.avro"))
>>> exoplanets = list(exoplanetsIter)
>>> print len(exoplanets)
1103

Next, build a scoring engine directly from PrettyPFA. This engine finds the coldest planet orbiting each star in the exoplanets dataset, with contingencies for missing data.

>>> engine, = prettypfa.engine('''
types:
  Planet = record(
    name:          string,                // Name of the planet
    detection:                            // Discovery technique
      enum([astrometry, imaging, microlensing, pulsar,
            radial_velocity, transit, ttv, OTHER]),
    discovered:    string,                // Year of discovery
    updated:       string,                // Date of last update
    mass:          union(double, null),   // Mass over Jupiter's mass
    radius:        union(double, null),   // Radius over Jupiter's
    period:        union(double, null),   // Planet year (Earth days)
    max_distance:  union(double, null),   // Distance from star (AU)
    eccentricity:  union(double, null),   // (0 = circle, 1 = escapes)
    temperature:   union(double, null),   // Temperature (Kelvin)
    temp_measured: union(boolean, null),  // True if the measured
    molecules:     array(string)          // Molecules observed
  );

  Star = record(
    name:    string,                      // Name of the star
    ra:      union(double, null),         // Right ascension (degrees)
    dec:     union(double, null),         // Declination (degrees)
    mag:     union(double, null),         // Magnitude (unitless)
    dist:    union(double, null),         // Distance away (parsecs)
    mass:    union(double, null),         // Mass over Sun's mass
    radius:  union(double, null),         // Radius over Sun's radius
    age:     union(double, null),         // Age (billions of years)
    temp:    union(double, null),         // Temperature (Kelvin)
    type:    union(string, null),         // Spectral type
    planets: array(Planet)                // Orbiting planets
  );

  PlanetWithTemp = record(planet: Planet, temp: double)

input: Star
output: Planet

method: emit
action:
  var star = input;  // name the input for convenience

  // build up a list of planets with temperature estimates
  var pt = json(array(PlanetWithTemp), []);
  foreach (planet: star.planets, seq: true) {
    var temp =
      ifnotnull(t: planet.temperature)
        // if a planet's temperature is already defined, use it
        t
      else {
        // otherwise, estimate it from the star
        ifnotnull(t: star.temp,
                  r: star.radius,
                  d: planet.max_distance) {
          var r_in_km = r * 695800.0;
          var d_in_km = d * 149600000.0;
          t / (d/r)**2
        }
        else
          // third case: not enough data to make any estimate
          null
      };
    // if the above resulted in an estimate, add it to the list
    ifnotnull(t: temp) {
      pt = a.append(pt, new(PlanetWithTemp,
                            planet: planet,
                            temp: t))
    }
  };

  // if the list is not empty...
  if (a.len(pt) > 0) {
    // find the coldest planet
    var coldest =
      a.minLT(pt, fcn(x: PlanetWithTemp,
                      y: PlanetWithTemp -> boolean) {
        x.temp < y.temp
      });

    // and emit it as the result of this scoring engine
    emit(coldest.planet)
  }
''')

Now run the scoring engine on the exoplanets dataset.

>>> def emit(x):
...     print x
...
>>> engine.emit = emit
>>>
>>> for star in exoplanets:
...     engine.action(star)
...
{u'discovered': '2014', u'updated': '2014-03-06', u'name': 'Kepler-207 d', u'temp_measured': True, u'period': 5.868075, u'detection': u'transit', u'eccentricity': None, u'radius': 0.295, u'molecules': [], u'max_distance': 0.068, u'mass': None, u'temperature': None}
{u'discovered': '2004', u'updated': '2012-05-25', u'name': 'HD 89307 b', u'temp_measured': True, u'period': 2199.0, u'detection': u'radial_velocity', u'eccentricity': 0.25, u'radius': None, u'molecules': [], u'max_distance': 3.34, u'mass': 2.0, u'temperature': None}
...